In a heat exchanger of the above mentioned type, the expansion of each tube end portion has the general purpose of ensuring that the header plate will be retained on the bundle of tubes, with the additional function of contributing to the sealing of the junction between the tube and the header plate.
The expression "substantially circular" in this specification is to be understood to mean that the holes formed through the header plate, and therefore the cross sections of the tube end portions, are circular in shape, or else they have a shape which is very close to being circular. In this latter case, the shape may be oval or elliptical, being characterised by a major axis of symmetry and a minor axis of symmetry, and being such that the ratio between the length of the major axis and the length of the minor axis is equal to at least 1.2.
In a heat exchanger of the kind described above, sealing between each tube end portion and the header plate is usually ensured by means of a sealing gasket, which is made of an elastomeric material and formed with compressible collar portions, each of which is compressed between the tube end portion and a further collar portion which is part of the header plate and which bounds the corresponding hole in the latter.
In a modification, it is possible to obtain the required sealing by means of a brazed joint, although this type of joint is more often used in the case in which the tube end portions are of flattened or elongated shape, being therefore very different from the circular shape.
In heat exchangers of the known types mentioned above, the substantially circular end portion of each tube is formed with an expanded mouth of matching shape, that is to say the expanded mouth is substantially circular; it extends over the whole periphery of the tube end portion. In the case in which the tube end portion is received in a respective one of the said holes with a sealing gasket interposed, the dimensions of the expanded mouth are generally close to those of the hole. The operation of making such an expanded mouth over the whole periphery of the end portion of the tube gives rise to a risk of creating cracks or tears in the latter, which is detrimental to sealing of the heat exchanger.
In order to overcome this disadvantage, it has been necessary up to the present time, especially in the case where the tubes are of aluminium alloys, to ensure that they have a specific metallurgical condition. This increases the selling price of the heat exchanger.
In the case of heat exchangers in which the end portions of the tubes have a non-circular cross section which is markedly different from the shape of a circle, these end portions are flattened or elongated and have two opposed long sides, which are straight or curved. It is known to provide an expanded mouth in such a case, but the known way of providing such a mouth is to form two localised deformations, which are provided respectively on the two opposed major sides of the end portion of the tube. This is for example described in the specifications of French patents Nos. 1 157 417, 2 181 497 and 2 525 337.
Moreover, the provision of expanded mouths in that particular form is not suitable for tubes having end portions having an elongated cross section which is substantially different in shape from a circle.